Social Darwinism was a phenomenon of the post-US civil war era: I suspect that the decline in black status in America in the post-revolutionary era had more to do with Upton Sinclair's remark, "It is difficult to get a man to understand something, when his salary depends upon his not understanding it " [1] - and there was an awful lot of money in slavery, especially after the cotton gin came in.
[1] And its inevitable corollary, "It is easy to get a man to believe the most outrageous shit, if his salary depends on his agreeing with it."
[1] And its inevitable corollary, "It is easy to get a man to believe the most outrageous shit, if his salary depends on his agreeing with it."